Rajendra Pachauri is the U.N.'s top climate scientist. He leads the Intergovernmental Panel on Climate Change (IPCC), which every five years produces the authoritative assessment of climate science. Its last report, in 2007, helped set the target of 450 parts per million (ppm) of carbon dioxide in the atmosphere, a target that many environmental groups and national governments have adopted as their goal for Copenhagen.

As many of you know, that number is out of date. When Jim Hansen and other scientists looked at phenomena like the Arctic ice melt of the last two summers, they produced new data demonstrating that 350 ppm is the bottom line. But it's been hard to get that news out to the powers that be. So today it comes as enormous and welcome news that Pachauri, from his New Delhi office, said that 350 was the number.

"As chairman of the Intergovernmental Panel on Climate Change, I cannot take a position because we do not make recommendations," said Rajendra Pachauri when asked if he supported calls to keep atmospheric carbon dioxide concentrations below 350 ppm. "But as a human being I am fully supportive of that goal. What is happening, and what is likely to happen, convinces me that the world must be really ambitious and very determined at moving toward a 350 target," he told Agence France-Presse in an interview.

Many national governments (and even some environmental groups) have stuck to a 450 ppm target—it seems politically "realistic." But Pachauri has taken away that gray area, and laid down the real bottom line. Physics and chemistry say 350, and that's that.

Pachauri cited the decision of the small island nations and less developed countries to endorse the 350 target. "I think this is a good development," he said. "Now people—including some scientists—see the seriousness of the impacts of climate change, and the fact that things are going to get substantially worse than what we had anticipated."

This news makes it much easier for all of us to push hard leading up to the 24 Oct "Day of Action" and the December Copenhagen climate talks. It's clear now that science is powerfully on the side of 350. Now we need the political world to follow suit.

• Bill McKibben, a scholar-in-residence at Middlebury College, is the author of a dozen books, most recently The Bill McKibben Reader. He serves on Grist's board of directors and is cofounder of 350.org.